Vehicle speed regulators or cruise controls, as they are commonly known, are generally separated into two types, the electromechanical type utilizing a speedometer-cable driven flyweight governor as a speed sensor and the electronic type which do not require a flyweight speed sensing means. Typically an electronic cruise control senses vehicle speed by measuring the output of a tachometer generator connected to the transmission output shaft or detecting the firing pulses from the engine ignition system. The electrically sensed vehicle speed is then electronically compared with the speed selected by the vehicle operator, and a control signal is generated having the characteristics thereof varied in accordance with such speed comparison. The control signal is applied to a servoactuator, usually of the pneumatically operated type, which servoactuator is connected to the vehicle throttle for moving same in response to the control signal. Several techniques for providing a control signal are known in the art and include a simple open loop comparison of the selected speed and vehicle speed by dividing speed error into incremental zones and providing predetermined control signal outputs as the speed error passes from one zone to another, an example of which technique is described in U.S. Pat. No. 3,869,019. A second type of electronic control signal is that using a governing equation in which the control signal characteristics are varied in accordance with successive computations of the values of the equation parameters using instantaneous values of vehicle speed, an example of which technique is described in the copending patent application, Ser. No. 469,568 filed May 13, 1974, now abandoned, in the names of G. L. Larson, Alberto Pi and M. W. Uitvlugt and assigned to the assignee of the present invention. Both of these aforementioned systems for providing a control signal by electronic techniques employ a control signal which comprises a series of pulses of substantially constant frequency whose width is modulated to indicate the signal intelligence.
The typical servoactuator employs a fluid chamber, the pressure of which is applied to an actuator, such as a flexible diaphragm, which moves in response to changes of pressure in the fluid chamber, and the actuator is adapted for connection to the vehicle throttle. The pressure in the fluid chamber may be controlled by a valve which alternates between a fluid pressure inlet and an exhaust port and, more commonly, a vacuum orifice and a vent orifice. The valve is thus of duty-cycle type operation and requires driving means responsive to the control signal for moving the valve. Typical speed control systems are usually activated by a switch in the form of a button depressed by the vehicle operator when the vehicle is travelling at the desired speed. The activation of the switch causes the control system to register the vehicle speed, at that point in time, as the selected speed at which the operator wishes the vehicle to be regulated. The selected speed or "SET" speed is then stored in the electronic memory of the control system and serves as a reference for continual comparison of the vehicle speed therewith. Such an arrangement is illustrated and described in the aforementioned copending application, Ser. No. 469,568 filed May 13, 1974.
However, as often occurs, the vehicle operator may wish to increase or decrease vehicle speed only a slight amount to accommodate changes in traffic and road conditions. In order to change the selected or governed speed of the system, it has heretofore been necessary to disable the control system as, for example, by tapping the brake pedal to cut the power supply to the controller by opening a series switch contacting the brake pedal and then manually accelerate or decelerate the vehicle to the desired new speed and reset the control system at this new desired speed. Such operations require that the driver take his eyes off the road and watch the speedometer until the vehicle has been brought to the new desired speed and the controller can be reset. This has proved to be annoying and also involves an element of hazard in diverting the driver's attention from traffic to the speedometer. Thus, it has long been desirable to provide some means of indexing the "SET" or control speed of a vehicle to accommodate minor changes in driving conditions, without requiring the vehicle operator to remove eyes from the road and divert his attention to the speedometer.